Air fractionating cycle and apparatus



Jan. 27,, 1953 c. J. SCHILLING AIR FRACTIONATING CYCLE AND APPARATQS Filed June 18, 1947 CLA RENCE J. SCHILLING INVENfO/P TORNE'V Jan. 2?, 1953, c. J. SCHILLING AIR FRACTIONATING CYCLE AND APPARATUS .2 sm'rs-srmm 2 Filed June 18, 1947 dw 23052 mi uni CLARENCE J.SCHILLING v TOR YQLLLN. Mm

ATTORNEY Patented Jan. 27, 1953 AIR FRACTIONATING CYCLE AND APPARATUS Clarence J. Schilling, Allentown, Pa., assignor to Air Products, Incorporated, Emmaus, Pa., a

corporation of Michigan Application June 18, 1947, Serial No. 755,286

2 Claims. 1

This invention relates to the manufacture of oxygen by the fractionation of atmospheric air and includes an operating cycle and an apparatus adapted to put the cycle into effect.

A purpose of the invention is to provide method and means for the manufacture of oxygen on a very large scale.

A purpose of the invention is to provide an apparatus and an operating cycle of extreme simplicity, with attendant minimization of first cost and operating expense.

The invention will be described with reference to the attached drawings, which will be understood to be flow sheets in which apparatus elements are indicated by symbols and wherein:

Figure 1 is a diagrammatic representation of an air fractionating system illustrating principles of the present invention with switching heat exchangers in which the air supply is refrigerated with returning nitrogen and oxygen components, and

Figure 2 is a diagrammatic representation of a different air system illustrating the principles of the present invention with switching accumulators.

Referring to the drawing, air enters the system at l and is substantially freed from dust in an air cleaner i l This element may be an electrostatic precipitator, a scrubber or a simple air filter. It is not essential to remove the dust completely, but only the coarser particles which might cause abrasion in the compression unit.

The cleaned air passes at I! (valve l3 being closed) to a compression unit consisting of a steam turbine or other power source H, a first and a second stage turbo-compressor l5 and IS, a water-cooled intercooler IT and an aftercooler H8.

The compressed air leaves the aftercooler via conduit I 9 at about 100 pounds absolute and at a temperature about 300 Kelvin. This conduit is branched at 20, about 80% of the air supply passing to a header 2| and thence to the nitrogen interchangers and about 20% to header 22 and thence to the oxygen interchangers.

The system includes a pair of switching" or deriming heat interchangers 23A-23B which are used in parallel for cooling the larger portion of the air supply 'by heat interchange against the cold gaseous nitrogen produced by the column. Each of these units consists generally of a shell 24, a gaseous nitrogen passage consisting of a plurality of tubes 25 and an auxiliary gas passage illustrated as an external gas jacket 28.

The upper ends of the interchangers communicate with the air inlet manifold 2| and with a nitrogen outlet manifold 21, which in turn is vented from the system through a nitrogen vent pipe 28, through a pair of reversing valves 29A- 29B. These valves are reciprocated through quarter turns, in synchronism and at suitable intervals, by means not shown. With these valves in the position shown in the figure, the right end of air manifold 2| is closed and air passes from the left end of the manifold through valve, 29A to a conduit which is branched at 30 to deliver air through conduit 3i to the tubes of interchanger 23A and through conduit 32 to the shell of interchanger 23B. In this position the left end of nitrogen manifold 21 is closed and the right end is in communication with the shell of interchanger 23A through conduit 33 and with the tubes of interchanger 23B through conduit 34, these conduits connecting at 35. When the valves are simultaneously reversed in position, as by a quarter turn clockwise, the functions of the described conduits are reversed, conduits 3| and 32 carrying vent nitrogen and conduits 33 and 34 carrying entering air.

The coupling of the interchangers in such manner that each divided stream flows always through the tubes of one interchanger and the shell of the other is important in avoiding variations in resistance to flow of low pressure gaseous nitrogen which often accompany valve reversals when a pair of interchangers are so connected that the flow of nitrogen is directed first through two sets of tubes and then through two shells.

The lower ends of the interchangers are coupled in a similar manner to manifolds which alternately convey air and nitrogen. Thus, manifold 38 is branched at 31 to the tubes of interchanger 23A and at 38 to the shell of interchanger 23B. Manifold 39 is oppositely branched, i. e., at 40 to the shell of interchanger 23A and at 4| to the tubes of interchanger 233. These manifolds are also branched at 42 and 43 to opposite sides of a flap valve 44, and at 45 and 46 to opposite sides of a flap valve 41.

With the reversing valves ISA-29B in the positions shown, manifold 36 is conveying air under relatively high pressure while manifold 39 is conveying nitrogen at a much lower pressure. The overbalancing pressure in branch 42 swings the flap'of valve 44 to the right, as illustrated, preventing the air from entering the opposite manifold through branch 43 and directing it into conduit 48 which leads to the fractionating column. The flap in valve 41 being pivoted below its center line, the excess pressure in branch 45 tips it 3 to the right, as illustrated, ail'ording a passage for nitrogen from conduit 48, leading from the fractionating column, into branch 88 and manifold 39 which passes gas upwardly through the interchangers.

The oxygen interchangers BOA-88B are structurally identical with the nitrogen interchangers above described except for the omission of the jackets 26. Air from the compression unit passes irom manifold 22 through reversing valve BIA and branch conduits 52 and 53 to the shell of interchanger 50A and to the tubes of interchanger 50B and through bottom connections 54 and 55. manifold 56, branch 51, flap valve 58 and conduit 58 to air conduit 48 leading to the column. Oxygen in gaseous form, withdrawn from the fractionating column through conduit 60, flows through flap valve 6|, manifold 62 and branches 88 and 84 to the tubes of interchanger 50A and the shell of interchanger 50B, escaping at the upper ends of the exchangers through conduits 65 and 86 to valve SIB, manifold 61 and product oxygen delivery pipe 68.

The fractionating column generally indicated at 89 may be any conventional or preferred twostage column. In any case it consists of a high pressure section 10 and a low pressure section H separated by a partition plate and a refluxing nitrogen condenser 12. Each of the sections is provided with bubble plates 18.

Liquid crude oxygen collecting in a pool 14 in the base of the high pressure section passes through a conduit 18 and an expansion valve I6 to an interchanger 11 in which it is in counterflow heat interchange with high pressure liquid nitrogen, the expanded crude oxygen then passing through conduit 18 to an intermediate point in the low pressure section.

The high pressure liquid nitrogen collecting in pool I! below the nitrogen condenser passes through conduit 80 to the opposite side of interchanger II in which it is cooled and stabilized by the expanded crude oxygen, flowing thence through expansion valve 8| and conduit 82 to the upper end of the low pressure section.

Gaseous low pressure nitrogen is withdrawn irom the top of the column through conduit 83. flowing to a jacket 84 surrounding a part of the air feed line 48, this jacket discharging into conduit 49 above referred to as leading to the nitrogen interchangers.

Oxygen in a desired state of purity, ordinarily 95% or over, collects over the head 88 of condenser and flows through conduit 88 to form a pool 81 surrounding tubes 12. Boiling in this space in condensing high pressure nitrogen vapor within the tubes, the oxygen vapor travels through bypass 88 to the vapor space above head 85, from which it is withdrawn through conduit 60 and the air-nitrogen interchanger 84 to the oxygen interchangers as above described.

The interchangers are operated in the cus tomary manner, the warm air passing through one side of each unit in counterflow to one of the cold product gases until the air passages become sufllciently fouled, by the accumulation of water ice and solid carbon dioxide, to give rise to a high pressure differential or to fall below a predetermined heat transfer efllciency. At this point the reversal of the valves causes the air stream to flow through the passage previously occupied by the cold gas, and which is clean, while the gas stream flows through the passage previously oc cupied by air, vaporizing and removing the ice and carbon dioxide snow.

It is a well known drawback to this procedure that the total products of fractionation flowing through a cold accumulator or its functionally equivalent deriming interchanger do not completely and dependably remove the accumulation of carbon dioxide snow and water ice from the surfaces on which they are deposited, and that such substantially complete removal may be effected by passing through the interchanger a quantity of cold gas materially greater than the quantity of air from which these deposits are accumulated.

To provide complete deriming for long period operation, it is essential that the cold end temperature difference between incoming air and purging product be 5 C. or less. To accomplish this, it is necessary to compensate for the higher specific heat of air under pressure especially at lower temperature. Adding quantity to the eiiluent product makes this possible by bringing the temperature-enthalpy curves of the counterflowing gases into approximate parallelism.

It is not necessary that the excess cold gas be in contact with the deposited solids, but only that it be in heat interchange relation with them. In consequence there are numerous ways in which this compensation may be effected, in any interchanger, direct contact or tubular, in which the gas to be cooled and the gas to be heated flow alternately through the same passage.

In the operating cycle here described the oxygen interchangers 50A50B are provided with an excess of the cold gas by passing through them a smaller quantity of air than that which corresponds to the quantity of oxygen produced, for example, say 20% of the total air supply instead of the 21% to 22% which would correspond with the oxygen yield.

The remainder or say of the air supply passes through the nitrogen interchangers 23A- 233 and the excess of cold gas is provided by nitrogen withdrawn in gaseous form from the high pressure section of the column, heated by passing through the nitrogen interchanger, cooled by expansion and returned at low pressure to pass again through the interchanger with the low pressure nitrogen taken from the top of the column, thus passing twice through the step of interchange.

In more detail, a suiflcient quantity of gaseous nitrogen, which may for example be perhaps 20% of the total nitrogen content of the air fractionated, is withdrawn from the dome of the column condenser 12; carrying with it any incondensible gases which might otherwise tend to accumulate there. The withdrawn gas, at about 100 pounds absolute and about 100 K., passes through conduits 88, and Si and is equally divided between the auxiliary gas passages 28 of the nitrogen interchangers, in which its temperature is raised to about 145 K. by interchange with entering warm air. These streams, which flow continuously through the two interchangers in parallel and constantly from the cold to the warm end, are collected in conduit 84 and pass through conduits and 88 to a turbo-expander 81. During normal operation, valve 88 in conduit 88 is open and valves 89 and I00 are closed.

In the expander 91 the pressure is reduced to about 24 pounds absolute in doing work and the temperature is thus reduced to about K. The expanded nitrogen stream then passes through conduit |0l to mix with the colder nitrogen stream passin through conduit 83, the temperature of the mixed nitrogen stream at the cold end of the interchangers being thus raised to about 96 K.

The withdrawal of as much as 20% of the total nitrogen made in this manner does not reduce the quantity of reflux liquid sufliciently to interfere with eiiicient operation of the low pressure column section, so long as oxygen of the highest purity is not required.

The expander 91 is coupled with a turbo-compressor I02 or other means for applying a power load. The compressor is desirable as providing a steady and readily controllable load. It is illustrated as taking air through conduit I03 and discharging it through a conduit HM controlled by a valve I05. If the oxygen produced by the column is to be delivered into a pipe line at a pressure above that available at the interchanger outlet. compressor I02 may be utilized for that purpose.

It is desirable to provide a cross-over line I06 to admit a controlled quantity of cold nitrogen into conduit 95 in case the temperature of the high pressure nitrogen passing from the jackets to the expander becomes too high. This quantity is controlled by regulation of valve 93.

It should be noted that the drawing shows only one turbo-expander 91. This unit, expanding the withdrawn high pressure nitrogen, sufflces to provide make-up refrigeration for the cycle but when of proper size for that purpose is insufllcient to provide refrigeration for starting up a warm apparatus. For this purpose it is desirable to provide the expander in duplicate or even in triplicate to ensure quick starts after a shut-down.

The operating cycle above described is advantageous over previously disclosed methods for controlling the temperature of the cold nitrogen entering the nitrogen interchangers, in doing away with the splitting of the air feed and with the introduction of air into the low pressure stage.

In methods heretofore proposed, a part of the air cooled in the main interchangers is diverted away from the high pressure section of the column through an interchange against a minor stream of product nitrogen passing to the interchangers or against the incoming air, this minor stream being then expanded and introduced into the low pressure column. This introduction lowers the efiiciency of fractionation in the low pressure section and the purity of the oxygen obtainable under given conditions, but is most ob- Jectionable in adding greatly to the difliculty experienced in regulating the operation of the column.

By applying the heating effect to a small part of the available high pressure nitrogen and diverting it to the interchangers without entering the column the regulation of nitrogen interchanger temperature is rendered wholly independent of regulation of column operation, and both are simplified without loss of refrigerative effect or interference with the most desirable column operating conditions. A further advantage in the described cycle lies in a material reduction in the size of the column required.

It will be seen from the above description that the air supply to the column is not passed through any step of drying by adsorption, water and carbon dioxide being removed solely by refrigeration. In starting up the apparatus, however, it is necessary to remove water by adsorption until the interchangers have cooled down to the temperature at which they will carry the dehydration 6 load, say to a cold end temperature of K.

To thus dry the entire air supply (thousands of cubic feet per minute) for even the few hours required to obtain interchanger cool-down requires a very large and costly drying installation, but I have devised a system by which the temperature may be reduced with the use of adsorption apparatus of much less size and capacity. This system functions as follows:

In the drawing, I01 indicates any conventional air drier, such unit consisting ordinarily of two shells filled with silica gel or activated alumina, through which the gas to be dried is passed alternately and from which the adsorbed water is driven out by a stream of heated air or other gas. As these drying units are in common use and well known, they are not illustrated but are indicated by a symbol. The adsorptive capacity of the unit may be 1% or less of that which would be required otherwise.

In starting up the warm apparatus, air at about 100 pounds absolute is delivered by the two-stage compressors through the nitrogen and oxygen interchangers. Valve I08 in conduit 48 being closed and valve 92 in conduit ill! open, the air does not enter the column but is directed to expander 91 through conduits H0, 90, I06, 95, and 96. Valves 99 and Hill are now opened and valve 98 is closed to direct all of the air stream through the small drying unit. As the air is to be recycled, the quantity to be dehydrated represents only one volume of the exchangers and piping.

The air expanded in 91 flows through conduit IOI into product nitrogen conduit 83 and into product oxygen conduit 60 through a cross-over I I I and an open valve I I2 and is thus divided between the two interchanger units. Valves 1 l3 and I may be provided in conduits 83 and 60 to exclude air from the column, though they are not strictly necessary.

In passing through expander 91 the temperature of the air'is somewhat reduced and the cold end temperature of the interchangers is gradually lowered. On leaving the interchangers, the dried air, now brought back to atmospheric temperature by interchange with the entering airstream, returns to the intake side of the compression unit through manifolds 21 and 61 and recycling conduit H5, valve [3 in this conduit being open and valves H6 and II! in the nitrogen and oxygen vents respectively being closed.

Except for make-up due to temperature drop, the same air is thus recirculated continuously through the steps of compression, interchange, drying and expansion. Thus the air within the systemwill be completely dehydrated before the interchangers come to working temperature, even with the use of a small drying unit. By bypassing the column during the drying-out operation the risk of carrying any solids into it is completely avoided. The removal of water'vapor obviates the formation of ice crystals and of attendant risk of damage to the expanders.

As is well known, it is possible for frozen hydrocarbons to accumulate at the bottom of liquid oxygen pool 8?, giving rise to risk of explosion. To avoid this risk it is desirable to withdraw continuously a small stream of liquid oxygen from the bottom of the pool, as through valve H8, and pass this liquid downwardly through a vaporizing coil H8 heated by the stream of crude oxygen flowing through conduit 16. The resultant oxygen vapor, carrying the volatillzed hydrocarbons,

passes through conduit IISA to join the stream of gaseous oxygen flowing through conduit 88.

Figure 2 illustrates certain alternatives to the procedure and apparatus already described:

(a) in the substitution of direct contact heat interchangers (the so-called cold accumulators) for the switching tubular heat interchangers of Fig. 1, a pair of accumulators being the full equivalent of a single switching interchanger;

(b) in the provision of means for making small quantities of pure oxygen as an adjunct to the described means for withdrawing hydrocarbons from the air fractionating column;

in the application of the unbalancing effect to both the nitrogen and the oxygen interchanger instead of to the nitrogen interchanger only.

These variants may be used in any combination, i. e., either or both interchangers may be unbalanced by the high pressure nitrogen cycle, and either or both may be of the tubular or of the accumulator type.

The modified plant illustrated in Fig. 2 has the same air supplying elements, numbered from I8 to I8 inclusive, as are shown in Fig. 1, and these need not again be described.

The air supply at a preferred pressure, which for example may be about 100# absolute, passes through conduits I28 and I2I to four reversing valves I22-I22-I24 and I28 which are functionally similar to valves 2I and 8I of Fig. l and which control the flow of gases into and out of the upper ends of direct contact heat interchanges ("cold accumulators) I28 and I2'I for product nitrogen and I28 and I28 for oxygen, these elements having the customary filling of metal exposing a large surface area.

With the valves in the position shown, air is flowing downwardly through interchangers I28 and I28 into conduit I28 by which it is passed into the high pressure section of two-stage column I2I. At the same time gaseous nitrogen flowing from the low pressure section of the column through conduit I22 is passing upwardly through interchanger I21 and through conduit I32 to a vent I34, while oxygen at a desired purity, as for example 95%, flows from the low pressure section of the column through conduit I38, passes upwardly through interchanger I28 and leaves the interchanger by way of conduit I28.

On moving the rotors of the control valves through 90 the gas flows are reversed, air passing downwardly through interchangers I21 and I28 while nitrogen flows upwardly through interchanger I28 and oxygen through interchanger I28. These reversals have already been described in detail in connection with Fig. l.

The interchangers are provided with secondary gas passages I2II2l which may be simple tubes or, like the primary passages I28-I28, may be filled to a desired height with heat conductive metallic elements.

Gaseous nitrogen is withdrawn from the high pressure section of the column through conduit I29 and is distributed by manifolds I48-I48 to the four secondary passages I2'|I2I. This flow is constant through the four secondary passages in parallel and in a direction opposite to that of air flow through the primary passages. The nitrogen streams. still at approximately the pressure carried in the high pressure section of the column, are collected in manifolds I4II4I and flow through conduits I42 to an expansion engine I42 which may well be a turbo-expander. This element is loaded by a compressor I44 which may conveniently be used to raise the oxygen product delivered to it through conduit I28 topipe line pressure.

In expander I42 the high pressure gaseous nitrogen is reduced to substantially the pressure carried in the low pressure column section, the expanded stream passing through conduit I48 to join the stream of gaseous nitrogen flowing through conduit I22 to the nitrogen interchangers I28 and I21.

Air fractionating columns producing large quantities of oxygen in gaseous form accumulate various hydrocarbons, mainly acetylene, at the bottom of pool 81 of liquid oxygen surrounding the nitrogen condenser.

It is customary to flush out these impurities occasionally, to avoid risk of explosion, but this practice is undesirable as causing wastage of oxygen.

In the structure shown in Fig. 2 the scouring of the liquid oxygen pool is accomplished without loss of oxygen by withdrawing continuously a small proportion of the oxygen output, say from 3% to 5% of the total quantity produced, from the bottom of the liquid pool, as at I48, into a correspondingly small single stage fractionating column I41. This column is provided with packing or plates in the usual manner and also with a refluxing condenser I48 in its upper end and a boiling coil I48 in its lower end.

A stream of high pressure air is drawn from conduit I28 through conduit I88 to the boiling coil and is expanded through valve I8I into the chamber surrounding condenser I48. From this chamber the expanded air passes through conduit I82 to Join the stream of crude oxygen passing to the low pressure column,

Pure liquid oxygen (containing traces of hydrocarbons) collects in the liquid pool surrounding the boiling coil and passes continuously through conduit I82 into an evaporating coil I84 enclosed in a shell I88. Heat for evaporating the pure liquid oxygen is supplied by a stream of crude oxygen drawn from the high pressure section of the column through conduit I88, the shell being vented into the chamber surrounding condenser I48 through conduit and expansion valve I81.

A mixture of nitrogen and oxygen is vented from the upper part of the small column, at a point below refluxing condenser I48, this product passing through conduit I88 to conduit I28 in which it is blended with the much larger quantity of relatively low purity oxygen passing to the oxygen interchangers I28 and I28.

The substantially nitrogen-free oxygen vapor produced in coil I84 passes through conduit I88 to a switching valve I88 by which it is directed alternately through passages I8I and I82 in heat interchange relation with the air flowing through interchangers I28 and I22. This valve is so timed that the flow of oxygen is through the interchanger through which air is flowing, the opposite passage being shut oil. The pure oxygen, returned substantially to atmospheric temperature and pressure, is vented from the system at I82.

It is desirable to pass air conduit I28, nitrogen conduit I22 and oxygen conduit I28 through an interchanger I84 in which the air stream is slightly cooled in imparting a relatively small amount of heat to the streams of product nitrogen and oxygen in advance of the main interchanger.

As in this modification oi the operating cycle the unbalancing eflect is applied to both pairs oi interchangers instead of to the nitrogen interchangers only as in Fig. 1, the warm air supply is divided between the two pairs oi interchangers in at least approximately the proportions in which the gaseous products are obtained irom the main column.

I claim as my invention:

1. The method of initiating an air fractionating operation which comprises: compressing a stream of air from the atmosphere and removing the heat of compression; adsorbing from said stream a portion oi its moisture content;

expanding and thereby cooling the partially dehydrated air stream: returning the expanded air stream in heat interchange relation with said compressed air stream and thence to said step oi compression as substantially the total air supply thereto; continuing the circulation of said air stream in a cycle through the steps aforesaid until the cold and temperature in said interchange step is sufllciently low substantially to deprive said air stream-oi moisture and carbon dioxide by consolation, and thereafter discontinuing said step of adsorption, admitting atmospheric air into said step of compression, and introducing said compressed and dehydrated air stream into said iractionating operation.

2. The method of initiating an air fractionating operation which comprises: compressing a stream of air from the atmosphere and removing the heat of compression, adsorbing from said stream a portion of its moisture content; expanding and thereby cooling the partially dehydrated air stream; returning the expanded air stream in heat interchange relation with said compressed air stream and thence to said step of compression as substantially the total air supply theretor continuing the circulation of said air stream in a cycle through at least the aforesaid steps oi compression, heat interchange, expansion, and return heat interchange, until the cold end temperature in the interchange step a is sufiiciently low substantially to deprive the air stream of moisture and carbon dioxide by congelation; thereafter readmitting air from the atmosphere into the step of compression, and introducing cooled air from which the moisture and 10 carbon dioxide have been substantially completely removed in the heat interchange step into the iractionating operation, the adsorption step being discontinued prior to the readmission of moisure containing air to the step of compression,

is and passing a product of the iractionating operation to the heat interchange step to cool the air therein.

CLARENCE J. SCHJLLING.

REFERENCES CITED The following references are of record in the file of this patent:

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